1. Field of the Invention
This invention relates in general to processes for producing gasoline octane enhancers, and more particularly, to processes for producing high octane gasoline blending components such as methyl tertiary butyl ether and tertiary butyl alcohol.
2. Brief Description of the Prior Art
In producing gasoline it has recently been desirable to use high octane blending components such as methyl tertiary butyl ether (MTBE) so that new blends of unleaded gasoline can be of sufficiently high octane rating. The typical process for producing MTBE catalytically reacts methanol with isobutene in a mixed C.sub.4 stream. The mixed C.sub.4 stream with isobutene serves as the feed for this process and is commonly derived from conventional fluid catalytic cracking units and coking operations present in crude oil refineries.
The process for catalytically reacting methanol with isobutene to form MTBE is a well-known process of etherification. In this process a stoichiometric excess of methanol is mixed with isobutene in the presence of an acidic ion exchange-type catalyst such as AMBERLYST-15. AMBERLYST-15 (Rohm and Haas) is a cationic, strongly acidic, ion exchange resin containing a sulfonated polystyrene cross-linked with divinyl benzene. An excess of methanol is used to attain a good conversion of isobutene and to suppress side reactions resulting in dimers, trimers and higher molecular weight polymers of isobutene. These products are of lower octane and reduce the desirability of the product.
A problem with all present etherifiction processes for producing MTBE is that the excess methanol needed to suppress production of less desirable products is itself a troublesome component of the reaction product. The excess methanol remains with the unreacted C.sub.4 's after MTBE separation and must be removed from the unreacted C.sub.4 's before further processing, for example, alkylation. The separated methanol is recycled to etherification or can be used elsewhere. This methanol separation and recovery typically requires a water wash tower, a methanol stripper, and corresponding pumps and heat exchangers. This separation is, therefore,undesirably expensive.
In some instances it is desirable to convert the isobutene to MTBE by etherification and to tertiary butyl alcohol (TBA) by hydration. Since TBA is a relatively high octane blending compound, the combined TBA and MTBE is a desirable gasoline blending stock. TBA can also be separated and used for other purposes. U.S. Pat. No. 4,423,251 shows a process for converting isobutene to both TBA and MTBE. More particularly, it shows an isobutene stream which is divided into two portions, the first of which is reacted for hydration to TBA. The second portion of the isobutene and remaining isobutene from the hydration process are introduced into an etherification zone. In this zone excess methanol is used to obtain a good conversion of the isobutene to the MTBE. Thus, the problem of separating excess methanol is present in this process in the same manner as other etherification processes.
It is therefore an object of the present invention to provide an improved process for producing a gasoline blending stock from isobutene. It is also an object of the present invention to provide such a process that produces a high octane combination of MTBE and TBA from isobutene and which is simpler and less expensive than previous processes. It is still further an object of the present invention to provide such a process which does not require separation of methanol following etherification.